As the density of devices formed in integrated circuit chips has increased, the problem of making reliable connections to the chip has become more complicated. Most integrated circuit chips include contact pads arranged around the periphery of the device so that electrical connections can conveniently be made by any of a number of bonding techniques. Some integrated circuits, however, have so many separate contact pads that it is not possible to locate them all around the chip periphery.
For example, known photonics systems may require an array of thousands of photodetectors, each requiring a separate connection, defined on a single integrated circuit chip having an area of only about one square centimeter. There are other integrated circuit devices in which the number of contact pads has become so numerous that one cannot make the required connections of the chip to external circuitry without difficult and painstaking effort. The U.S. Pat. of Thomas, No. 4,843,191, provides one approach to the problem comprising the use of several rows of contact pads on the periphery of the chip to be contacted by tape automated (TAB) bonding to conductors carried on different insulator layers. It is difficult to use this technique with a matrix array, such as photoconductor arrays.
There has therefore developed a long-felt need for techniques to make contact to a high density of contact pads of an integrated circuit chip, particularly a dense matrix array of pads, in a manner that is reasonably inexpensive, reliable, and which is consistent with the mass production of such devices.
The invention makes use of a material known as an anisotropic conductor material, that is, a material that will conduct electricity in only one direction. Such material may comprise an insulative sheet containing spaced conductors extending across the thickness of the sheet as described, for example, in the U.S. Pat. of Kashiro et al., No. 4,209,481, granted Jun. 24, 1980. As another example, it may comprise a sheet of polymer material containing conductive ferromagnetic particles which have been aligned during cure of the polymer by a magnet to form conductor columns connecting opposite surfaces of the polymer sheet, as described generally in the U.S. Pat. of Hechtman et al., No. 4,778,635, granted Oct. 18, 1988 and in the paper, "Applications and Reliability of the AT&T Elastomeric Conductive Polymer Interconnection (ECPI) System," J. A. Fulton et al., IEPS, September 1990, pp. 930-943, both of which are hereby incorporated herein by reference.